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How much pressure will this chamber take?

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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How much pressure will this chamber take?

Unread postAuthor: Dave_424 » Mon Jun 22, 2009 4:04 pm

Hey guys, I have a sodastream bottle that is pressure tested to 3600 psi

What X mix do you think I could use it to? The fittings will be 1/4" steel fittings

Dave
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Unread postAuthor: MountainousDew » Mon Jun 22, 2009 4:15 pm

Since the bottle has been tested to 3600 psi, it will not be a problem at high mixes. And with that said, you need to know the pressure rating for the steel fittings, because if anything would fail, it would be the fittings.
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Unread postAuthor: Dave_424 » Mon Jun 22, 2009 4:17 pm

yeah okay, i don't know the X mix to combustion pressure so I can't determent the X Number

Dave
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Unread postAuthor: MountainousDew » Mon Jun 22, 2009 4:20 pm

What you should do is download HGDT.
Enter in the info for your gun, and play around with mixes and figure out what pressure you will be dealing with.
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Unread postAuthor: daccel » Mon Jun 22, 2009 8:12 pm

MountainousDew wrote:...if anything would fail, it would be the fittings.


Not necessarily. You would need to know what percent of the failure pressure the 3600 psi pressure test on the bottle is.

See here for details on pressure testing of 1/2" fittings. If a 1/2" tee burst at 8700 psi, 1/4" would presumably be higher than that. Since he probably wouldn't be taking the mix above the rated pressure for the bottle, the fittings are probably fine.

That said, ymmv, so don't quote me if it goes boom :wink:.
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Unread postAuthor: Biopyro » Tue Jun 23, 2009 4:52 am

I just did a quick bit of guess work with the chamber in HGDT (6cm diameter, 20cm long?) 35x mix 35bar burst disk, mapp gas gives a peak pressure of 3335psi. So it can handle quite a bit!
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Unread postAuthor: Dave_424 » Tue Jun 23, 2009 6:48 am

That is awesome thanks.

The chamber is aprox 5cm x 25cm

However I am taping a hole in the end of the cylinder so I will reduce the X mix to around 15. I might also Use some of that metal putty to secure the fixig as well

Dave
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Unread postAuthor: inonickname » Tue Jun 23, 2009 6:59 am

Dave_424 wrote:That is awesome thanks.

The chamber is aprox 5cm x 25cm

However I am taping a hole in the end of the cylinder so I will reduce the X mix to around 15. I might also Use some of that metal putty to secure the fixig as well

Dave


If you can avoid it, don't. If you're good at tapping threads and all then go ahead. What size threads, tapered or untapered..etc btw?

Edit: If you tap a hole in the chamber please re-pressure test it via hydrotesting.
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Unread postAuthor: Dave_424 » Tue Jun 23, 2009 7:58 am

I will be taping a 1/4" BSP thread,

Working from a 15X mix, Max chamber pressure is around 1600 psi, because it is a 1/4" opening, 1600 devided by four gives 419 pounds it has to withhold.

The chamber is steel and I will be drilling the hole ready to tap it but I am sure that it would hold over 1000 pounds

I will hydrotest it with my airgun pump to about 2000 psi.

Dave
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Unread postAuthor: jeepkahn » Tue Jun 23, 2009 8:58 am

Dave_424 wrote:I will be taping a 1/4" BSP thread,

Working from a 15X mix, Max chamber pressure is around 1600 psi, because it is a 1/4" opening, 1600 devided by four gives 419 pounds it has to withhold.

The chamber is steel and I will be drilling the hole ready to tap it but I am sure that it would hold over 1000 pounds

I will hydrotest it with my airgun pump to about 2000 psi.

Dave


Try 78.5 pounds..... Surface area of a 1/4" is .049"sq=.049x1600=78.5

it's the change in structural integrity of the chamber that your gonna have to be careful of, the fitting will be fine but the hole for the fitting will compound any stresses to that portion of the chamber...

Make sure you check your math and calculations before proceeding, screwing up the math can make BAD things happen...
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Unread postAuthor: Dave_424 » Tue Jun 23, 2009 9:07 am

Yeah okay, thanks for that but good to know that I was okay with the 400 odd pounds but it is actually much less

I have drilled the hole in the end of the chamber and the wall thickness is is 10-11 mm Which I think will give enough threads to hold the fitting safely.

The botton of the chamber is rounded which is better than being flat but i do know that drilling a hole in the bottom will weaken the chamber.

For a 15X mix peak pressure is around 1600, I think that a 2000 psi pressure test is okay??? or would it be better to go with 2500??

Dave
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Unread postAuthor: jeepkahn » Tue Jun 23, 2009 9:33 am

I personally would take it to 4times the pressure your going to use it at...no less than double the pressure for safety sake...

I personally hate the words "pressure tested to"... true pressure ratings given to pressure vessels are well below the actual tested to strength, ie, nitrogen tanks are used at 4000psi but are rated at 12000, co2 tanks are rated at 3000psi but actual pressure is only 850.....

if you drilled in the center of the round and you drilled straight you should be good...
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Unread postAuthor: Dave_424 » Tue Jun 23, 2009 9:38 am

Okay then, i will test it to 3000-3500 psi. 3500 is the maximum of my pump
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Unread postAuthor: Gippeto » Tue Jun 23, 2009 2:04 pm

Okay, I have a couple points which I feel should at least be looked at.

1) I don't know the drill size used when tapping for 1/4"BSP, but for 1/4" npt, the drill size is 7/16". A fair bit bigger than the 1/4" that calculations have been based on thus far. Just check is all I'm saying.

2) The walls of the pressure cylinder are likely not thick enough to properly support threading. Consider brazing or welding half of a coupling union to the tank. Lots of threads.

3) Pressure testing using any compressed gas, is just plain DUMB ! :evil: DON"T DO IT. What do you think is going to happen in the event of a failure???

Buy a cheap grease gun and use that.
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Last edited by Gippeto on Tue Jun 23, 2009 11:34 pm, edited 1 time in total.
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Unread postAuthor: twizi » Tue Jun 23, 2009 2:10 pm

gippeto i think he would use water but he would force the water in the tank with air to compress the water
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